Broad molecular weight distributions have been thought to be necessary to achieve a high flexural modulus to achieve stiffness. Broad molecular weight distributions, however, are associated with polymers having high molecular weight fractions and low molecular weight fractions. The high molecular weight fraction (sometimes referred to as a high molecular weight tail) can lead to increased die swell while processing the polymer. This die swell will be especially pronounced for processes that utilize low melt flow rate (MFR) polymers, especially fractional melt flow rate polymers. The low molecular weight fraction (sometimes referred to as a low molecular weight tail) can lead to high xylene solubles. Also, the low molecular weight fractions can cause processing problems, such as die drip and smoking during polymer processing, as well as environmental problems in the form of volatile organic emissions. Smoking is of particular concern with processes which utilize very high melt flow rate polymers, such as fiber spinning and nonwoven fabrics production. Increasing melt flow rate has been associated with decreasing molecular weight which decreasing molecular weight would be expected to lower impact strength.
Additionally, lower molecular weight distribution has typically been associated with polypropylenes having lower stiffness, as measured by flexural modulus (see, for example, “Polypropylene Handbook”, E. P. Moore, pg. 243, Hanser/Gardner Publications, Cincinnati (1996)). What is desired is a polypropylene that exhibits a relatively narrow molecular weight distribution, but also exhibits high flexural modulus and low xylene solubles.